Photocatalytic Activity of Ti-SBA-15/C3N4 for Degradation of 2,4-Dichlorophenoxyacetic Acid in Water under Visible Light

In the present study, the photocatalytic activity of Ti-SBA-15/C3N4 catalysts was investigated to degrade 2,4-Dichlorophenoxyacetic acid (2,4-D) herbicides in water under visible light irradiation. The catalysts were synthesized via a simple hydrothermal method and characterized by various analytical techniques, including SAXS, N2 adsorption-desorption isotherms, Zeta potential, PL, FT-IR, XRF, TGA, and UV-DRS. Our study indicated that the 2.5Ti-SBA-15/C3N4 had higher efficiency in the degradation of 2,4-D than Ti-SBA-15 and C3N4. The decomposition of 2,4-D reached 60% under 180 minutes of visible light irradiation at room temperature on 2.5Ti-SBA-15/C3N4. Moreover, the degradation of 2,4-D on Ti-SBA-15/C3N4 was pseudo-first-order kinetics with the highest rate constant (0.00484 min−1), which was much higher than that obtained for other photocatalysts reported recently. Furthermore, the catalyst can be reused at least two times for photodegradation of 2,4-D solution under visible light irradiation within a slight decrease in catalytic activity.


Introduction
Last century, 2,4-Dichlorophenoxyacetic acid (2,4-D) was one of the most generally used herbicides in agricultural producing countries [1,2]. 2,4-D has primarily been used in agriculture to control weeds in corn and grain, forestry, and lawn care practices [3]. Still, its harmful effects are severe, such as harsh contaminants of superficial and ground waters due to its slow degradation by biological processes, affecting amphibious life in water bodies, causing chromosomal aberrations in human lymphocytes [4,5]. erefore, the development of methods to remove wastewater contaminants has gained considerable interest in commercial and academic worldwide. However, most current techniques for the degradation of 2,4-D, such as photo Fenton oxidation and microbial degradation, have to be carried out in a complicated process, producing hazardous by-products or expensive costs [6][7][8].
Recently, photocatalysis has been a promising advanced water treatment process to replace or support current methods [9]. Titanium-based catalyst has been considered one of the most popular photocatalysts to degrade persistent organic pollutants such as 2,4-D and lindane. Among those, Ti-SBA-15 has been widely used as a catalyst, photocatalyst, and catalyst support for other materials because of its highly ordered and uniform mesoporous channels, thick silica walls, and large surface area [10]. However, several studies indicated that Ti-SBA-15 had limited practical application under visible light conditions because it only absorbed UV light [11,12]. On the other hand, graphitic carbon nitride (g-C 3 N 4 ), a metal-free material with a low electronic bandgap falling in the visible light range of the solar spectrum, has some advantages in photocatalytic applications such as chemically stability under ambient conditions, flexible optical, and non-toxic [13,14]. g-C 3 N 4 was also modified by various semiconductors (e g., Ag 2 S, CdS, B 4 O 5 I 2 , Ag 3 PO 4 , Ni 3 C@Ni, ZnO, Bi 2 O 3 -V 2 O 5 ) or element doping (N, P, S) to improve its photocatalytic degradation of dye or hydro evolution [15][16][17][18][19]. To the best of our insight, there are no  reports on using Ti-SBA-15 modification with carbon nitride  for the photodegradation of herbicides 2,4-D under visible  light irradiation. To develop more visible light-efficient catalysts, it is crucial to design photocatalytic systems that are able to operate effectively [20]. Herein, Ti-SBA-15 loading with carbon nitride was prepared and studied for an efficient photocatalyst for the degradation 2,4-D under visible light at neutral pH. e relationship between absorption amount and the photodegradation of 2,4-D on Ti-SBA-15/C 3 N 4 was discussed in detail. Furthermore, the 2,4-D degradation processes on Ti-SBA-15/C 3 N 4 have also exchanged views about the kinetics and mechanism in this study.

Synthesis of xTi-SBA-15.
Ti-SBA-15 materials were synthesized by a simple method following the previous reports published anywhere [21]. In an exemplary process, 4.0 g of pluronic P123 was dissolved in 100 mL Deionized (DI) water and 20 ml hydrochloric acid (37 wt.%) under stirring at 40 o C. After that, 8.0 g of TEOS was added dropwise and stirred for 2 hours. e required amount of Titanium isopropoxide in 10 mL ethanol was dropped slowly into the solution under vigorous stirring at 40 o C for 24 hours. After aging at 90 o C for 48 h under steady-state conditions, the as-synthesized material was filtered, washed with DI water, and dried in an oven at 70 o C for several hours. e material was then calcined at 550 o C for 6 h in air and denoted as xTi-SBA-15 (x is the % wt Ti � 0; 1.0; 2.5; and 5.0).

Synthesis of xTi-SBA-15/C3N4.
A certain amount of melamine, urea, dicyandiamide, and xTi-SBA-15 was ground together. is mixture was added to 2 mL deionized water in one neck round-bottom flask under stirring and heating until 85 o C for 2 hours. Next, water was removed from the mixture by evaporation. Finally, the solid was heated at 550 o C for 2 h in the inert atmosphere to give a yellow powder denoted as xTi-SBA-15/C 3 N 4 .

Characterization Methods.
e FT-IR spectra were obtained using a Bruker ALPHA FT-IR spectrometer. UV-DRS spectra were measured using a PerkinElmer Lambda 365 spectrometer with a reflectance sphere using pure calcium sulfate (CaSO 4 ) powder as a white reflection standard. Small-angle X-ray scattering (SAXS) measurements were carried out using a Kratky-type instrument (SAXSess, Anton Paar, Austria) operated at 40 kV and 50 mA in slit collimation using a two-dimensional CCD detector cooled to -40 o C. Standard powder X-ray diffraction (XRD) patterns were collected on a Panalytical X'Pert diffractometer (40 kV, 40 mA) equipped with an Xcelerator detector using automatic divergence slits and CuKα1 radiation (λ � 0.15406 nm) in a 2θ scan range between 10 and 50°. X-ray fluorescence (XRF) data have been collected using a Panalytical Epsilon 1 spectrometer operated with an Ag X-ray source. N 2 adsorption-desorption measurements were carried out at -196 o C using an ASAP 2020. e zeta potential was analyzed on the ZS90/Malvern system at neutral pH. e photoluminescence (PL) spectra were measured on the HORIBA Jobin Yvon-Fluoromax-4. ermal gravimetric analysis (TGA) was measured on the Labsys TG/DSC1600 instrument (TMA Setaram, France) with a heating rate of 10 o C min −1 in air.

Degradation of 2,4-D.
In a typical experiment, 10 mg catalysts were suspended in 30 mL of the 2,4-D solution with the concentration of 20 mg/L and kept for 90 minutes in the absence of light under stirring to reach adsorption-desorption equilibrium. en, the mixture was placed into a photoreactor and irradiated by a 1000 W Xenon lamp (LAX 1000, Oriel) equipped with a 90°deflection reflector system (MS 90) containing a dichroic mirror. e distance between the reactor and the deflection reflector system was 12 cm. Experiments were performed with a UV cut-off filter (λ ≥ 420 nm, the light intensity: 130 mW cm −2 ). During the irradiation, 2 mL of the liquid was continuously taken from the mixture reaction at a 30 minutes interval for the 2,4-D degradation analysis by High-Performance Liquid Chromatography (HPLC) with the UV-detector using a Lichrosorb RP 18 column (Merck). In addition, a pseudofirst-order model was applied to the study of the degradation reaction kinetics. Figure 1 shows the SAXS patterns of xTi-SBA-15 and samples modified with C 3 N 4 . Ti-SBA-15 Figure 1(a) has a sharp diffraction peak at q � 0.67 nm −1 (2 θ � 0.94°) due to (100) reflections of a hexagonal mesoporous lattice. In addition, small peaks were also observed at 1 nm −1 < q < 1.5 nm −1 (1.4 o < 2 ? < 2.1 o ), corresponding to (110), (200) reflections of the mesopores in this pattern. ese results indicate the formation of a hexagonally packed porous structure. For xTi-SBA-15, all features for hexagonal lattice were observed without their slight decrease in peak intensities.

Characterization of xTi-SBA-15 and xTi-SBA-15/C3N4.
e respective q values were also decreased from 0.67 nm −1 to 0.621 nm −1 along with the increase of the Ti amount. e unit cell parameter was 10.8 nm for pure SBA-15 and increased in the presence of Ti in the porous structure. e difference in the unit cell parameter indicates the larger size of the Ti heteroatom incorporated into SBA-15. It suggested that the long-range order of the mesoporous structure of SBA-15 has remained [22]. Moreover, the SAXS patterns of all xTi-SBA-15/C 3 N 4 also show hexagonal diffraction peaks due to reflections of (100), (110), and (200) without a slight decrease in intensity. Figure 1(b), indicating that the structure of these materials has remained after modification. e XRD of xTi-SBA-15/C 3 N 4 (x � 0, 2.5) were also measured at wide-angle and shown in Figure 2(a). e results showed that both materials presented a broad peak at 2θ � 27.5 o , corresponding to the (002) plane of C 3 N 4 within the decreasing intensity, suggesting that the structure of C 3 N 4 has also remained after modification by xTi-SBA-15. e N 2 adsorption-desorption measurements also confirmed the information on the mesoporous structure Figure 2(b). e typical type IV isotherms were observed with large H1 hysteresis loops at a high relative pressure of 0.6-0.8, corresponding to capillary condensation of nitrogen in the uniform mesopores. Table 1 and Figure 2(d) show the pore structure character parameters of the synthesized materials. e surface area, pore-volume, and pore size of Ti-SBA-15 increase slightly compared to SBA-15. It could be due to Ti atoms replacing Si atoms and expanding the SBA-15 structure [23,24]. On the other hand, the surface area decreased quickly after modification due to the presence of C 3 N 4 within the mesopores Figure 2(c). e amounts of Ti in xTi-SBA-15 (x � 1, 2.5, and 5) were 0.9%, 2.5%, and 4.7%, respectively, determined by XRF method.
Electronic and optical properties are of fundamental importance to determine the photoactivity of a catalyst. UV-DRS spectroscopy was applied for characterizing the optical properties of the catalysts. e weak peak is observed at 256 nm for all the samples xTi-SBA-15/C 3 N 4 and pure C 3 N 4 , attributed to the aromatic ring's p ⟶ π * Figure 3(a). Besides, the intense peak at 380 nm corresponding to the n ⟶ π * transitions is caused by electron transfer from nonbonding nitrogen orbital to an aromatic anti-bonding orbital. e absorption band of xTi-SBA-15 is slightly toward long-wavelength than C 3 N 4, as well as an increase in the absorption of visible light. e Tauc plots Figure 3(b) show the bandgap of xTi-SBA-15 (2.73 eV, 2.72 eV, and 2.72 eV, respectively) slightly decreased in comparison with pure C 3 N 4 (2.74 eV). e C 3 N 4 bandgap value corresponds to -1.13 eV, 1.59 eV of the conduction band (CB), and valence band (VB). It is hard to confirm the CB and VB of Ti-SBA-15; however, it is assumed that they were near the TiO 2 value (-0.5 eV, 2.7 eV of CB and VB, respectively). From these results, it could be demonstrated that the promoted xTi-SBA-15/C 3 N 4 materials are more active than C 3 N 4 under visible light irradiation.
Photoluminescence (PL) spectra were also measured to analyze photogenerated electron-hole pairs' migration, transfer, and recombination process in the photocatalysts (Figure 4). e emission peak of all materials was at around 470 nm, indicating the structure of C 3 N 4 has remained after modification. e peak intensity of xTi-SBA-15/C 3 N 4 decreases along with the increase of Ti amount. It is noticed that the PL intensity of 2.5Ti-SBA-15/C 3 N 4 was the lowest, corresponding to lower recombination of electron-hole pair.
e FT-IR spectra of these materials are shown in Figure 5.
e results indicated that the carbon nitride replicas show many peaks due to carbon nitride networks' formation by condensing precursors, which can be assigned for C-N heterocycle at 1426 cm −1 , 1320 cm −1 , and 1243 cm −1 [25]. Journal of Analytical Methods in Chemistry asymmetric stretching vibration, and the peaks at 810 cm −1 can be allocated to the symmetric stretching and distortion forms of Si-O-Si [26]. Additionally, the band at 960 cm −1 is generally an identity of Si-O-Ti bonds' existence, as seen in Figure 5.   Journal of Analytical Methods in Chemistry corresponding to the water loss in the pore of 2.5Ti-SBA-15. en C 3 N 4 in the composite decomposed entirely at high temperature (>600 o C), and 7.5% weight corresponds to the remaining 2.5Ti-SBA-15 ( Figure 6). However, the reaction yield is relatively low, less than 10% compared to C 3 N 4 (42%) (the data not shown).

Photodegradation of 2,4-D over
e results in Figure 7   , OH . radical or hole (h + ) of heterojunction of Ti-SBA-15/C 3 N 4 can oxidize 2,4-D to degradation products like CO 2 and water, as presented in Figure 8.
To further study the reaction kinetics of the 2,4-D photodegradation by xTi-SBA-15/C 3 N 4 , the photodegradation data were adapted by implementing a pseudofirst-order model as given by Ln(C 0 /C) � −kt , where k is the  Journal of Analytical Methods in Chemistry apparent first-order rate constant (min −1 ); C 0 is the initial concentration at t � 0, and C is the concentration at time t of the 2,4-D solution. Figure 9 shows the linear relationship between ln(C 0 /C) and the irradiation time of photodegradation of 2,4-D on the catalysts. It was clearly observed that the photodegradation curve in all cases matched smoothly with pseudo-first-order kinetics. In addition, the apparent rate constants k of all the catalysts were estimated and decorated in line with the model (Figure 9). e rate constants k values were significantly increased with the rise of Ti loading. e 2.5Ti-SBA-15/C 3 N 4 catalyst displayed the highest rate constant (0.00484 min −1 ) for 2,4-D degradation, which was much higher than that obtained for other photocatalysts reported recently as ZnO-FeY [29], PdO/ Al 2 O 3 -Nd 2 O 3 [30], chitosan-TiO 2 beads [31], Fe/TiO 2 [32] or photo-Fenton process [33].
To optimize the effect of 2,5Ti-SBA-15 in the composite for 2,4-D photodegradation, numerous composites with different amounts of 2,5Ti-SBA-15 were also prepared. Figure 10 shows the decomposition of 2,4-D, and Figure 11 displays the relationship between ln(C 0 /C) and the irradiation time of photodegradation depending on the amount of 2,5Ti-SBA-15 in target materials. e photodegradation of 2,4-D under visible light irradiation is increased from 50% (k � 0.00397 min −1 ) to 60% (k � 0.00484 min −1 ), coincident with the rise of 2,5Ti-SBA-15 from 2% to 7.5%. However,    when 2,5Ti-SBA-15 increases up to 10% wt, the photodegradation of 2,4-D slightly decreases, whereas the absorption amount was highest. e decreased absorption of visible light may cause it.
Finally, the recycling ability of the 2.5Ti-SBA-15/C 3 N 4 catalyst was investigated for degradation of the 2,4-D solution under visible light irradiation. e experiments were also carried out using the same procedure without taking the liquid from the mixture reaction. After 180 minutes, the solid was collected by centrifugation and washed with water several times. en, the solid was dried at 100 o C and used for the next reaction. e results showed that 2.5Ti-SBA-15/ C 3 N 4 is recycled at least two times with slight losing catalytic activity. e decrease would be ascribed to the loss of catalyst during experiments (Figure 12). From these results, it is demonstrated that the photocatalyst is robust and stable.

Conclusions
In an effort to develop photocatalysts, this study reported the catalytic activity of Ti-SBA-15/C 3 N 4 for the degradation of 2,4-D under visible light irradiation. ese synthesized materials were characterized by various analytical methods such as SASX, XRD, UV-DRS, N 2 adsorption-desorption isotherms, Zeta potential, XRF, PL, TGA, and FT-IR. e photodegradation of 2,4-D over Ti-SBA-15/C 3 N 4 is much higher than pure Ti-SBA-15, SBA-15/C 3 N 4 , or C 3 N 4 . e reaction proceeded to give excellent efficiency on 2.5Ti-SBA-15/C 3 N 4 with 60% degradation 2,4-D under visible light at room temperature for 180 minutes. e photodegradation of 2,4-D was pseudo-first-order kinetic with the highest constant (k � 0.00484 min −1 ), much higher than that obtained for other photocatalysts reported recently. In addition, the catalyst can be reused at least two times within a slight decrease of photocatalytic activities. It was realized that Ti-SBA-15 was an absorber and electron migration exists in the heterojunction of Ti-SBA-15/C 3 N 4 to enhance the photocatalytic activity of the catalyst. Since a good correlation was established between the adsorption capacity and amount of Ti in Ti-SBA-15/C 3 N 4, 2.5Ti-SBA-15 was determined to improve the adsorption property for 90 minutes.

Data Availability
e data used to support the findings of this study are included in the article.  57315854) and the Federal Ministry for Economic Cooperation and Development (BMZ) inside the framework "SDG Bilateral Graduate school program to support equipment and apparatus. Sincerely, thank Dr. Norbert Steinfeldt, Leibniz Institute for Catalysis, Rostock, Germany, who helps us measure XRF and SAXS patterns; it is hard to complete this manuscript without his support.